As far as I know Casey's little sister has not been shown or named yet. I think that since she could be an important character in the 2012 show that it would make more sense to re-imagine a character from different versions of TMNT then make here up completely. I think that the best choices to have as his sister are Shadow Jones or Angel Bridge.
Shadow Jones
Shadow in the comics is Casey's adopted daughter. This can easily be changed to his sister or having her being blood related. She could keep her basic story without any big changes. She could also be shown as any age since in the comics she grows from a baby to a young adult. This would be good for the creators because that would have more freedom with the character. She seems to be closer to Raph and Mikey out of the turtles. I think that she is the easiest choice since she's already related to Casey and has story line that already exist that could be adapted for the new show. (look on tmnt wiki for more info)
Angel Bridge
Angel is in 2003 series and the IDW comics. She is a purple dragon who Casey treats as a friend and as a sister. It could be hard to use her since a main part of her character is that she at some point was a Purple Dragon. But she has already been in different versions of TMNT which makes her a more staple character. She would have to be a teenager since in both versions that is the age she is portrayed as. Her ethnicity seems to vary as well which is confusing. She also seems to be closest to Raph and Mikey I think that she would probably be harder to do than Shadow but she is more recent and has been in more universes. (look on TMNT wiki or watch episode 'Fallen Angel' for more info.
What do you think?
Who do you think would be the best?
Are there any other characters that could have been her?
Or has she already be shown and I have done this for nothing?-If so tell me about her
Tell me what you thank and plz comment but not trolls.